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Copy and Paste This Permanent Link:http://www.waterefficiency.net/may-june-2009/one-size-doesnt.aspx

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Photo: Xerxes Corp.

Increasingly, onsite water storage is becoming a more integral part of water conservation. In many locations and situations, local regulations are requiring commercial facility owners to take responsibility for treating wastewater onsite and providing their own water sources for some applications. In both the commercial and residential sector, many owners are looking for ways to manage the environment and a finite resource—as well as reap the financial benefits of conserving water.

Several systems and tank materials are available, and one thing is for sure: No one system or material is ideal for every water conservation situation. Owners need to consider major factors such as storage volume and factors affecting the structural integrity of the tank when choosing a system.

Water Storage of All Shapes and Sizes
Paul Eldredge, assistant public works director for the City of Brentwood, CA, reports that three large concrete storage tanks the city has had constructed in the past few years are key components in its water conservation plans. Brentwood is located in the East Bay area of California and has a climate that is more like the state’s Central Valley than the rest of the Bay Area, he points out. “We have long, hot, dry summers, and our irrigation use increases in the summer. We’ve got some San Francisco Bay–Delta winds that can kick up periodically, so the combination of hot, dry, and windy conditions can dry out the ground quickly.”

Eldredge explains that Brentwood is separated from the coastal area by a small mountain, “So, we don’t get a lot of the coastal fog and dampness you’d get if you were closer to the coastal area,” he says.

In 2004, the city had the first of two, 4-million-gallon, partially aboveground, concrete tanks constructed. Eldredge notes that the first tank was to be located in a residential neighborhood and appearance was important. Given the necessary volume, the city researched a prestressed concrete tank building system from DYK Incorporated that is designed to prevent leakage via a method of putting concrete into compression, a state in which it offers its maximum structural integrity. For the sake of appearance, the tank was given a façade of architectural stone. A second concrete tank was to be constructed partially underground, into a hillside, two years later. Eldredge reports that the tank would serve as a retaining wall as well as a water storage structure, considerably narrowing the cost difference between steel and concrete, the tank material ultimately selected.

Photo: Containment Solutions Fiberglass is competitive with concrete in the 2,500- to 10,000-gallon range and is growing its presence in the 10,000-gallon-plus segment.

The city had its most recently constructed tank, a 1.5-million-gallon structure at its water treatment facility, built out of the same prestressed system about two years ago. This structure has inner and outer tanks, the inner one for final disinfection and the outer one for storage. All three of the city’s concrete tanks are tied into its water treatment system.

Eldredge says that he and the city settled on the prestressed concrete building system after conducting thorough research and making site visits to look at other DYK tanks in service. He adds that one factor that made concrete particularly attractive was its performance in seismically sensitive areas such as the Bay Area. “All of the information we had received led us to believe that [the tanks] were structurally sound and the prestressing was a tried and true technology—it wasn’t some new product off the street,” he says.

The experiences of Michael Stark, president of Stark Environmental LLC, Columbia, PA, underscore the fact that some materials are better suited to some water storage situations than other materials. Stark’s company designs and installs multiple storage systems, including the Roth MultiTank from Roth Global Plastics Inc. This system is primarily designed for rainwater collection and storage in residential applications—which, obviously, have smaller capacity requirements than the City of Brentwood. Stark notes that polyethylene tanks better suit smaller-capacity underground rainwater storage applications than some other materials.

Underground rainwater storage systems, Stark points out, merit the consideration of two factors that necessitate a tank with high structural integrity and polyethylene provides sufficient durability in many cases. One factor is the stability of the soil, the other the height of the water table. A high water table can put the tank in contact with deleterious groundwater. A low-profile tank not only keeps it above the water table, but also allows shallower excavations, which are beneficial from a safety standpoint and can reduce construction costs. Stark, whose company serves the Mid-Atlantic region, notes that much of the US population lives in coastal areas where these soil and water table issues are common relative to water storage system installation.

Concrete and polyethylene are two of the materials commonly used for water storage tanks. Various materials that are used have relative benefits and are prevalent in situations that lend themselves to these benefits.

Different Applications, Different Materials
Steel is a tried-and-true material for storage tanks, owing to its competitive cost, Stark points out. “Obviously, without some coating, you run a lot of potential for rust developing on the inside of the tank,” he says. “And on the outside, being in contact with the ground, there could be deterioration if it doesn’t have some kind of coating on it. No doubt, steel tanks are strong, but they’re fairly heavy” and cannot be moved into place by a pair of workers, he adds.

Photo: Tnemec Company Inc. The cost of coatings is a major maintenance consideration for elevated or partially elevated tanks, as on this steel tank in Rosemont, IL.

One characteristic of the polyethylene MultiTanks that appeals to Stark, he says, is the fact that they incorporate a high percentage of recycled material. Additionally, “Polyethylene, as plastics go, it’s considered a pretty benign plastic in that it doesn’t impact the liquids stored in it, and, typically, it also doesn’t bleed things into the soils around it and doesn’t get attacked by things in the soil.”

Stark’s company also installs fiberglass tanks. He argues that the advantage of this material over steel is lighter weight, and, like polyethylene, it does not need an interior lining. However, fiberglass is more cost-effective in larger sizes, he adds. Stark points out that precast concrete tanks have a strong presence in larger sizes, yet some sites are not suitable for truck and crane access, making polyethylene a better fit. Also, he points out, the price competitiveness of concrete depends somewhat on the availability of quality aggregates in various regions. A durability issue in concrete to consider is cracking and repair of alternative materials such as polyethylene is easier, he adds. Finally, although recycled concrete is commonly used in both precast and site-cast structures, cement production is characterized by high-energy use and high emissions, so some might consider materials like polyethylene to be more sustainable. Concrete becomes more economically attractive at larger volumes, he concludes.

Peter Young, vice president of sales and technical services for fiberglass tank manufacturer Containment Solutions Inc., identifies specific water storage tank capacities that various materials suit. Polyethylene tanks have a strong presence up to 2,500 gallons; above this capacity, greater thickness is required. Concrete is particularly strong in the 2,500- to 10,000-gallon range and traditionally dominated the market in the 10,000-gallon-plus segment. However, Young says, fiberglass has been competitive with concrete in the smaller range and is growing its presence in the larger range, because it becomes more cost-effective with scale. Although Containment Solutions’ individual tanks have a 50,000-gallon-size limit, he points out that several can be combined using flange fittings on the bottoms and manifold pipes that connect the tanks. Steel has traditionally dominated larger capacities of 250,000 gallons and more. Noting that steel has been a dependable material for years, he cautions that the interior of the tank should have a liner to maintain water quality.

Polyethylene was not considered for the city of Brentwood’s water conservation strategy because of its capacity needs, says Eldredge, who also acknowledges the cost-effectiveness of steel. Still, Eldredge says, the initial cost difference between steel and concrete is narrower than in the past. “The primary disadvantage [of steel] is that we have to periodically go in there and inspect the coatings or the interior of the tank and there’s a lot of [operations and maintenance] and capital costs associated with recoating those tanks and inspecting them and with concrete we do not have that challenge,” he says, adding that the city also has four large steel storage tanks.

The choice of concrete came down to a lower life-cycle cost, Eldredge says. “For the first tank we did, the main selling point was that we were trying to install this tank; we wanted to have a [partially buried tank to minimize its visual impact. If there were a significant cost savings to steel, we would have looked at it more closely. Concrete is so much more flexible because you can bury it, partially bury it, or completely bury it and build stuff on top of it. A steel tank is just a tank, and you try and berm it and plant trees and hope that nobody notices it. I’ve come to really like the concrete tank system—it provides a lot more options for engineers and planners. As space becomes more scarce, we’re putting tanks in areas where people don’t want them, and you can put these into existing environments with minimal visual impact. When all is said and done, it’s the visual impact that people are going to care about because people are going to see the tank every day.”

An advantage of elevated steel tanks is their use of gravity, which eliminates the need for pumps to move the water, points out Doug Hansen, director of water tank market sales for Tnemec Co. Inc., a manufacturer of industrial coatings used on structures such as water storage tanks. About the only competitor to elevated steel tanks are a composite structure that have been built since the 1970s, i.e., steel tanks supported by concrete shafts—designed to eliminate the need for painting the shaft—Hansen notes. Tom Bloomer, P.E., regional business development manager for DYK, argues that concrete tanks are much better suited to underground structures. DYK’s prestressing system makes the concrete in its tanks highly ductile to handle seismic and static stresses, Bloomer adds. For tanks up to 500,000 gallons, conventional reinforced concrete typically is used for water storage, but, above this volume, prestressing is necessary to handle the stresses on the walls, according to Bloomer.

The DYK tanks constructed in Brentwood have flat concrete membrane floors, cast-in-place walls, and column-supported two-way flat slab roofs. The tank wall is vertically post-tensioned with all-thread threadbars and is circumferentially prestressed using galvanized seven-wire strand and incorporates a flat, two-way slab concrete roof. The prestressing take the concrete out of tension—a state of being pulled apart—where it is weakest, while tensioning the steel puts that material in its strongest state. This construction method, combined with the tanks’ circular shape, eliminates stress concentrations and uniformly distributes loads around the tank circumference. In such large structures relying upon a material that is susceptible to cracking, the steel tensioning is essential, according to the company. Additionally, Bloomer points out, concrete tanks are subject to differential temperature and dryness loads because the tank walls are wet on the inside and dry on the outside, and the temperature varies between the two sides. To further protect the galvanized steel strands, the system utilizes the automated application of a one-and-a-half-inch shotcrete cover coat; the automated application is used to ensure uniform thickness. The company reports that these tanks have retained their structural integrity amid major earthquakes such as the 1971 San Fernando Valley, 1989 Loma Prieta, 1994 Northridge, and 2001 Washington State earthquakes.

Photo: Xerxes Corp. Flexibility is a major factor for many storage projects.

Photo: Cal Aerial Imaging The prestressed concrete tanks—a 2.2 million-gallon potable unit and a 1.8 million-gallon recycled water tank—use compression for structural integrity.

The roof and floor are separated from the corewall by neoprene bearing pads, a design intended to provide an unrestrained connection and reduce bending moments induced by hydrostatic, thermal, backfill, and seismic forces. This “free-sliding” connection at the wall base and wall top is designed to enhance the seismic performance of the tank by allowing the floor, wall, and roof to act independently and a continuous PVC waterstop between the floor and wall is used to ensure a watertight joint. Alternatives are available for the roof. A concrete, two-way, flat slab roof has a slight upward slope to the tank’s center. A grid of equally spaced round columns supports the flat roof. Free-span concrete or aluminum dome roofs can also be used and eliminate the need for interior columns, because the dome is entirely supported by the tank wall.

Bloomer says that the system offers owners construction flexibility. The tank can adapt aesthetically by incorporating finishes such as paint, stucco, or exterior insulation and finish systems (EIFS).

In contrast, the Roth MultiTank suits the residential-scale rainwater harvesting market sector and allows builders to earn credit toward the Leadership in Energy and Environmental Design (LEED) Green Building System. The system collects rainwater from a house’s gutters and filters out leaves and other contaminants with a two-stage process before the rainwater enters the underground polyethylene holding tank. The system suits configurations in which an irrigation system pump is located either inside the tank or inside the house.

In either pumping configuration, rainwater enters the storage tank through a “calmed inlet” that prevents the disturbance and re-suspension of fine sediments that migrate to the bottom of the tank, and introduces oxygen into the lower layers of the tank so as to prevent anaerobic conditions from forming. Once a maximum level is reached in the tank, an overflow siphon skims the water and removes lighter-than-water particles, maintaining quality water and allowing oxygen diffusion at the water surface. A pre-charged irrigation pressure tank dispenses the stored rainwater on a predetermined schedule. According to the company, capturing water on a 1,500 square-foot roof would allow a family to reduce its water bill by 50% and save about 35,000 gallons of water annually.

Joe Brown, vice president of sales and marketing at Roth Global Plastics Inc., discusses some unique design features of the MultiTank. This tank varies in size from 535 to nearly 1,800 gallons and is blow-molded by a computer-controlled, multi-layer machine. According to Brown, many polyethylene tanks are actually designed for use in septic systems and, therefore, are full all the time and continually exert hydraulic pressure on the soil. In contrast, the MultiTank does not rely on hydraulic pressure inside the tank, but, rather, largely owes its structural integrity to a thick horizontal rib located between each set of vertical ribs.

Containment Solutions’ fiberglass Flowtite Tank, which is often used to store rainwater and graywater treated by various recycling systems, is another water-storage system that reportedly allows contractors to earn LEED points. The manufacturer uses steel cylindrical steel molds called mandrels in fabricating the tank. The rotation of the mandrel and the application of the materials are controlled by a computer and as the mandrel rotates, resin, glass, and specially treated silica are precisely metered onto the mandrel. Using an instrument that magnetically senses the metal mold surface through the fiberglass-reinforced plastic laminate, the manufacturer measures the thickness of the laminate at many points throughout the surface of the tank wall.

Designed to enhance the durability of some water-storage tanks, Tnemec’s new HydroFlon fluoropolymer polyurethane coating reportedly has a life expectancy of 15 years or more and is formulated to retain gloss and color longer than traditional urethane topcoats. The company says that the coating is available in two-component and single component formulations and can be brushed, rolled, or sprayed. The product also has 60% volume solids and high-ultraviolet and saltwater protection, making it particularly suited to coastal environments.

Photo: Xerxes Corp. In the future, several forces will drive change in water storage, including materials and maintenance.

Any discussion of available tank materials must include steel. One company, the Tank Connection Affiliate Group, says that it provides all four types of steel tank construction: bolted rolled, tapered panel (RTP), field-weld, shop-weld, and hybrid tank (combining bolted flat-panel steel, field-weld, and concrete). Focusing on bolted RTP, the manufacturer reports that standard liquid tank capacities range from 25,000 to 8 million gallons. The company claims that the system’s structural integrity comes from its design, proprietary coating systems, and field installation procedures that utilize a synchronized, hydraulic screw jack process, allowing field crews to install tanks at grade level.

Another steel tank manufacturer, Columbian TecTank, designs and fabricates bolted and factory-welded tanks. Its environmentally controlled process uses continuous tunnel design, beginning with surface preparation followed by the coating application, and final thermal curing at baking temperatures up to 400°F. The company also uses a proprietary epoxy powder coating for interior surfaces that provides corrosion resistance and immersion performance, as well as flow, impact, and abrasion-resistant properties. The company points out that the system conforms to NSF Standard 61 for potable water. Columbian also has an exterior coating system that includes a special primer for corrosion resistance with a urethane topcoat for color retention.

The exteriors of the company’s factory-welded tanks are coated with a special primer and a urethane or a zinc base/epoxy intermediate/performance urethane finish topcoat for high-corrosion resistance and color retention. The elimination of field welding reduces erection costs, according to the company. Standard tank capacities range from 4,000 to 3 million gallons, and material options include factory-coated carbon steel, galvanized steel, stainless steel, and aluminum. Roof options include 1:12 sloped steel, aluminum geodesic dome, and open top with wind girder.

According to Tom Tietjen of Xerxes Corp., Fiberglass (FRP) underground tanks are rapidly gaining favor among water collection/conservation system designers and owners for the variety of advantages they offer over the traditional products that have been used. While competitively priced or less expensive to install, Tietjen points out that FRP tanks provide unmatched corrosion resistant properties not found with materials in alternative (concrete or steel) tanks. Additionally, they are a lightweight, factory-produced product that can be shipped one piece in very large capacities (12-foot diameter and up to 65,000 gallons) directly to a jobsite and installed easily and quickly.

FRP tanks provide an excellent, long-term, 30-plus-year track record of watertight performance, having become the standard for the retail petroleum industry where storage of hazardous materials underground is taken very seriously. Tietjen notes that today FRP tanks are routinely being specified for a wide variety of water and wastewater applications, some associated with Green Building designs such as rainwater collection or plumbing graywater systems. Projects for water with potable applications can be specified with an NSF listing, explains Tietjen, assuring the specifier of compliance with the strict NSF standard. FRP tanks are available from manufacturing facilities located throughout North America.

Forces Driving Change
Experts identify several forces that will drive change in water storage in coming years, from applications to materials to maintenance.

Young reports that Containment Solutions has identified three major growth markets for its system: fire suppression, decentralized wastewater treatment, and rainwater harvesting. He says that the company supplies many tanks for dedicated sprinkler use in a building, and that some municipal building codes now require a dedicated supply and not the municipal supply in some situations. Similarly, some new commercial buildings are not allowed to utilize a municipal wastewater treatment system and must treat the water onsite instead. This market sector also encompasses the car wash industry, which uses large volumes of water. Rainwater harvesting seems to be growing by the week, Young argues. For now, the focus is on reusing captured rainwater for irrigation, but he also can see rainwater being used for toilets in the future. He adds that water storage is a growing component of companies’ business; 10 years ago, it accounted for only about 1–2% of business, and now it makes up about 25%.

Brown also sees rainwater harvesting as a source of growth for Roth, albeit gradual. He adds that, in North America, water is still plentiful and inexpensive enough that economic drivers of growth do not yet exist as in other parts of the world.

Bloomer of DYK notes that storage capacities have consistently increased in recent years, adding that his company was recently involved in construction of a 40-million-gallon tank.

In terms of system materials and design, Hansen says he thinks that steel tanks will still be prevalent, but owners will increasingly seek ways to prolong their service lives, and that a major steel tank rehabilitation market does and will exist. Owners will put a lot of emphasis on durability-enhancing coatings in this market, because their cost accounts for 15–25% of a tank’s maintenance costs. In terms of dollars, he says, coating a 500,000-gallon tank might cost $140,000, so owners favorably view any potential extension in recoating. Also, according to Hansen, taking a tank out of service might take five to 15 weeks.